Circuit and method for driving an array of light emitting pixels

ABSTRACT

A technique for driving a column of pixels that include light emitting elements. The technique incorporates feedback data provided from feedback data sources connected to the data line and to feedback line of the array, pixel driving circuit with feedback path. The technique can also include block of the reference elements for input signal corrections.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application of U.S. ApplicationSer. No. 10/554,754, filed Oct. 28, 2005, now U.S. Pat. No. 7,978,187B2, which is a U.S. national phase of International Application No.PCT/CA2004/001742, filed Sep. 23, 2004, which claims the benefit ofpriority of Canadian Patent Application No. 2,472,689, filed Jun. 29,2004, and Canadian Patent Application No. 2,443,206, filed Sep. 23,2003, each of which are incorporated by reference in their entirety.

COPYRIGHT

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patentdisclosure, as it appears in the Patent and Trademark Office patentfiles or records, but otherwise reserves all copyright rightswhatsoever.

FIELD OF INVENTION

The present invention generally relates to a light emitting devicedisplay techniques, and more particularly, to a technique for drivinglight emitting elements that uses a feedback architecture duringprogramming to compensate for instability and non-uniformity of thepixels.

BACKGROUND OF THE INVENTION

Recently active-matrix organic light-emitting diode (OLED) displays havebecome more attractive due to advantages over conventional liquidcrystal flat displays. These advantages include the ability to fabricateOLED displays at a relatively low cost and high efficiency. Further thedisplays do not require backlighting and provide a wide viewing angle.

An active-matrix organic light-emitting diode (AMOLED) displaycompromises an array of rows and columns of pixels, each having an OLEDand some active devices such as thin film transistors. Since OLEDs arecurrent driven devices the pixel circuit of an AMOLED should be capableof providing an accurate and constant drive current to achieve aconsistent and uniform luminance.

As disclosed in U.S. Pat. No. 5,748,160, a simple pixel circuitcomprises two thin film transistors (TFTs) and an OLED. In this circuit,the OLED is connected to the drain terminal of a driving TFT and a gateterminal of the driving TFT is connected to a column line through aswitching TFT. A storage capacitor connected between the gate terminalof the driving TFT and ground is used to maintain the voltage at thegate terminal of the driving TFT when the pixel circuit is disconnectedfrom column line. For this circuit the current through OLED stronglydepends on the characteristic parameters of the driving TFT. Since thecharacteristic parameters of a TFT, particularly, the threshold voltageunder bias stress, vary by time, and such changes may differ from pixelto pixel, the induced image distortion may be unacceptably high.

One of the methods that has been employed to make the current drivingcircuit less sensitive to the shift in the threshold voltage isprogramming the pixel with current instead of voltage. In this method,the OLED current is less dependent on the voltage-currentcharacteristics of driving transistors. Implementations of currentprogrammed pixel circuits for OLEDs have been disclosed e.g. Yi HE etal., “Current Source a-Si: H Thin-Film Transistor Circuit for ActiveMatrix Organic Light-Emitting Displays”, IEEE Electro Device Letters,Vol. 21, No. 12, p 590-592, December 2000). A drawback of the currentprogramming method is that it is slow, particularly for low programmingcurrent levels, due to the large line capacitance. As a result, voltageprogramming methods are desirable considering their speed. This isparticularly true for large area TVs and displays.

Another method to make the drive current less sensitive to transistorparameters is to use current feedback. United States patent application20020101172A1 provides a driving system with current feedback. Anexternal current comparator compares the pixel current with a referencecurrent and generates an appropriate signal to control the pixelcurrent. One drawback of the disclosed method is that the control signalis a current, which can limit the programming speed. Another drawback ofthe method is that the anode and cathode electrode of each OLED have tobe patterned, which creates reliability concerns in the currently usedOLED fabrication process.

Luminance feedback is another method that has been used to stabilizeOLED luminance. As described in U.S. patent application 20030151569feedback readout circuits responsive to the feedback signal representingthe light output of the OLED can be used to provide brightness control.A drawback of the disclosed method is that every pixel requires aphoto-sensor that is optically coupled to the OLED. This results inintegration issues. Another drawback is that the low level of thefeedback signal generated by a photo-sensor may lead to the poorsignal-noise ratio, thereby narrowing the dynamic range of the system.

SUMMARY OF THE INVENTION

The present invention provides several driving circuits having afeedback control-system architecture that can be used for driving acolumn of the light emitting devices and are suitable for use in SMOLEDdisplays. In the present invention, a feedback voltage is generated byan on-pixel feedback circuit or element. This voltage is used to adjustthe programming voltage of the pixel.

According to an aspect of the invention each pixel in the column isconnected to the feedback-type control unit via signal line and feedbacklines, and receives a scanning clock signal via select line connectionterminal. The programming voltage applied to the pixel through thesignal line sets the driving current through the light emitting element.The programming voltage can be accurately adjusted by an externalcontrol unit through the use of feedback voltage generated by theon-pixel feedback circuit. The feedback voltage is proportional to thedriving current of the light emitting element and is used to set theprogramming voltage so as to achieve the desired driving current despitepresence of any instability (shift in characteristics of transistors andlight emitting element) and non-uniformity across pixels

The column control unit may be connected to the block of referenceelements formed on the display substrate in order to correct an error inthe output current level caused by inaccuracy of the pixel components ortemperature drift. The block of reference elements may also include aphoto-sensor optically coupled to the light emitting element in order toprovide a luminance feedback compensation for brightness variationsinduced by instability of organic material or temperature changes.

According to another aspect of the invention a pixel circuit for use ina display is provided. The display comprising a plurality of pixels witheach pixel having a select line, a signal line, a feedback line. Thepixel circuit comprising a light emitting element, a drive part forproviding a drive current to the light emitting element, the drive parthaving a storage capacitor and a switch use transistor having a gateterminal connected to the select line, and a first terminal connected tothe signal line, and a second terminal, and an on-pixel feedback elementfor generating a feedback voltage representing a drive current providedto the light emitting element, the feedback signal being provided to thefeedback line.

According to another aspect of the invention a pixel circuit for use ina display is provided. The display comprising a plurality of pixels witheach pixel having a first select line, a second select line, a signalline, a feedback line. The pixel circuit comprising a light emittingelement, a drive part for providing a drive current to the lightemitting element, the drive part comprising a storage capacitor, aswitch use transistor having a gate terminal connected to the firstselect line, a first terminal connected to the signal line and a secondterminal, a drive use transistor having a gate terminal connected to thesecond terminal of the switch use transistor, a first terminal and asecond terminal connected to the light emitting element, and an on-pixelfeedback circuit for generating a feedback voltage representing a drivecurrent provided to the light emitting element. The feedback circuitcomprising a resistor connected between the second terminal of saiddrive use transistor and a potential, and a feedback transistor having agate connected to the second select line, a first terminal connected tothe first terminal of the drive use transistor and a second terminalconnected to the feedback line.

According to another aspect of the invention a pixel circuit for use ina display is provided. The display comprising a plurality of pixels witheach pixel having a select line, a signal line, a feedback line. Thepixel circuit comprising a light emitting element, a drive part forproviding a drive current to the light emitting element, the drive partcomprising, a storage capacitor, a switch use transistor having a gateterminal connected to the select line, a first terminal connected to thesignal line and a second terminal, a drive use transistor having a gateterminal connected to the second terminal of the switch use transistor,a first terminal and a second terminal connected to the light emittingelement, and an on-pixel feedback circuit for generating a feedbackvoltage representing a drive current provided to the light emittingelement. The feedback circuit comprising a resistor connected betweenthe second terminal of said drive use transistor and a potential, and afeedback transistor having a gate connected to the select line, a firstterminal connected to the first terminal of the drive use transistor anda second terminal connected to the feedback line.

According to another aspect of the invention a display device isprovided. The display device comprising a select line, signal line towhich a voltage signal in accordance with both brightness and feedbackinformation is supplied, a feedback line to which a feedback voltagesignal in accordance with current level of drive current is supplied, aplurality of pixels forming an array of pixels, each pixel of theplurality formed on a substrate at an intersecting portion of saidscanning line and said signal and feedback lines, each pixel comprisinga light emitting element, a current driving circuit having a storagecapacitor and a switch use transistor, and a feedback circuit to providefeedback signals representing a current output of said current drivingcircuit, a display column control circuit for receiving input signals,adjusting the input signals using a reference circuit formed on thesubstrate at each column, and modifying the input signals in response tothe feedback signals from pixels in the column to produce a desiredbrightness level of said light emitting element in a selected pixel, anda selecting line drive circuit for successively activating selectinglines.

According to another aspect of the invention a method of driving aplurality of light emitting elements arranged in a column at a desiredbrightness is provided. The method comprising the steps of selecting onepixel of a plurality of pixels in the column, establishing the desiredbrightness of a reference light emitting element by adjusting areference current flowing through the light emitting element in responseto a photocurrent from a photo-sensor that is optically coupled with thereference light emitting element, converting the reference current intoa corresponding voltage level, transmitting the voltage level to theselected pixel, converting the voltage level into a drive current andgenerating a feedback signal representing a drive current level,adjusting the voltage level in response to the feedback signal from theselected pixel to establish a drive current substantially equal to thereference current, storing the adjusted voltage level, and driving thelight emitting element with the drive current in accordance with theadjusted voltage level to produce the desired brightness level in thepixel.

Advantages of the present invention include the ability to provide astable current to the light emitting diode over time, therebymaintaining image quality. Moreover, the combination of the externalcurrent feedback for pixel programming and luminance feedback for datasignal preprocessing provides brightness control and compensationdespite instability and non-uniformity in pixels. The circuits occupy asmall area and are voltage programmed with voltage feedback. The use ofvoltage for programming and feedback improves the programming speed,which is necessary for large area displays and TVs.

This summary of the invention does not necessarily describe all featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1 is a block diagram of an example of the configuration of adisplay device with feedback control-system architecture according to anembodiment of the invention;

FIG. 2 is a block diagram of pixel architecture according to anembodiment of the invention;

FIG. 3A is a circuit diagram of a pixel circuit and column control unitaccording to an embodiment of the invention;

FIG. 3B depicts the corresponding waveforms for the circuit of FIG. 3Aaccording to an embodiment of the invention;

FIG. 4 is a circuit diagram of a modification. of the embodiment of FIG.3A;

FIG. 5 is a schematic of a pixel circuit for a common cathode OLEDconfiguration according to an embodiment of the invention;

FIG. 6A is a circuit diagram of a column control unit and a pixelcircuit having p-channel type transistors according to an embodiment ofthe invention;

FIG. 6B depicts the corresponding waveforms for the circuit of FIG. 6Aaccording to an embodiment of the invention;

FIG. 7 is a circuit diagram of a column control unit and a pixel circuitwith a p-channel type transistor switch according to an embodiment ofthe invention;

FIG. 8 is a circuit diagram of a column control unit and a pixel circuithaving p-channel and n-channel type transistors according to anembodiment of the invention;

FIG. 9 is a circuit diagram of a column control unit and a pixel circuitwith a current mirror as current driving circuit according to anembodiment of the invention;

FIG. 10 is a circuit diagram of a modification of the embodiment of FIG.9;

FIG. 11 is a circuit diagram of a modification of the embodiment of FIG.3;

FIG. 12 is a circuit diagram of a pixel circuit, column control unit anda reference cell with implemented luminance feedback according to anembodiment of the invention;

FIG. 13 is a circuit diagram of a pixel circuit and column control unitwith a reference diode according to an embodiment of the invention;

FIG. 14 is a circuit diagram of a pixel circuit, column control unitwith a reference OLED according to an embodiment of the invention;

FIG. 15 is a circuit diagram of a modification of the embodiment of FIG.14.

DETAILED DESCRIPTION

The present invention encompasses a technique for driving of columns ofpixels where each pixel comprises a light emitting element, inparticular, an organic light emitting diode (OLED).

FIG. 1 presents a display device having a feedback control-systemarchitecture 10 and an array of addressable pixels 11. The pixels 11 arecontrolled by a select line driver 12 and data driver 13. As shown inFIG. 1, a separate feedback control unit 14 is provided on each columnline of the array. The feedback control unit 14 of a given column isconnected to each pixel in the column via a signal line 15 and afeedback line 16. A block of reference elements 17, located on thedisplay substrate, may also be provided. The block of the referenceelements 17 includes some elements of the pixel circuit for input signalcorrections and may also include a photo-sensor that is opticallycoupled to a light emitting element to implement a luminance feedback.

The structure of a given pixel 11, according to an embodiment of theinvention is shown in FIG. 2. As shown in FIG. 2, the pixel has an OLED21, a current driving circuit 22, controlled by the stored voltage levelusing a storage capacitor 23, a feedback circuit 24, and switches S1 andS2. The switches S1 and S2 can be any suitable switching device, but arepreferably an insulating gate type field effect transistor. The pixel 11operates in writing and hold modes. In the writing mode, when selectline (s) are activated, the switches S1 and S2 are turned on, and thecurrent driving circuit 22 receives the signal voltage from control unit14, while the on-pixel feedback circuit 24 feeds the voltage feedbacksignal. The driving current through the OLED 21 can thereby beaccurately controlled through the use of negative feedback. In the holdmode, the switches S1 and S2 are turned off and the driving circuit 22provides the driving current having a current level in accordance withthe voltage level to the storage capacitor 23.

FIG. 3A shows a pixel drive circuit according to an alternativeembodiment and a circuit diagram of the control unit 14. Controllingsignals are shown in FIG. 3B.

The pixel drive circuit comprises three transistors 34,36 and 38, aresistor 32, a storage capacitor Cs and an OLED 31. The pixel drivecircuit is connected to a select line, a feedback line, and a signalline. A power supply node having a positive potential Vdd and commonground are also shown.

Transistors 34,36 and 38 can be fabricated using amorphous silicon, polysilicon, appropriate organic semiconductors and NMOS or CMOStechnologies. The on-pixel feedback circuit is consisted of a thin filmresistor 32 that can be fabricated with any appropriate material andtechnology, which provides sufficient stability. For instance, inamorphous silicon technology the resistor 32 can be fabricated using N+amorphous silicon or N+ microcrystalline silicon.

The drain terminal of driving transistor 36 is connected to the cathodeof OLED 31. The source terminal of transistor 36 is connected toresistor 32 and the gate terminal is connected to the signal linethrough transistor 34. Resistor 32 is connected between the sourceterminal of transistor 36 and the common ground.

Transistors 34 and 38 are driving switch and feedback switchtransistors, respectively. The gate terminals of transistors 34 and 38are connected to the select line. The source terminal of transistor 34is connected to the signal line and the drain terminal is connected tothe gate terminal of transistor 36. The source terminal of transistor 38is connected to the feedback line and the drain terminal is connected toresistor 32. All OLEDs of the different pixels have a common anodeelectrode, connected to the voltage supply node (Vdd). Storage capacitorCs is connected between the gate terminal of transistor 36 and commonground. It can be connected between gate and source terminals oftransistor 36. For the latter, capacitor Cs can be implemented by thegate-source capacitance of transistor 36.

The external controlling unit 33 in its simplest form is a high-gain,low offset difference amplifier with a negative feedback connection.

During the writing mode, the select signal goes high, turning ontransistors 34 and 38. As a result, the driving transistor 36, alongwith the external difference amplifier 33 and resistor 32 make a circuitwith negative feedback. The difference in the voltage level between aninput signal voltage and a voltage drop across the resistor 32 isamplified by the difference amplifier 33, adjusting the potential on thegate of transistor 36. After the initial transients the output currentstabilizes and in the case of a high-gain feedback loop the currentpassing through the OLED 31 is:

$\begin{matrix}{I_{OLED} = {\frac{Vinp}{Rf}.}} & (1)\end{matrix}$During the hold mode, the select line goes low, so transistors 34 and 38are turned off and the pixel is disconnected. Since the gate voltage ofdriving transistor 36 is stored in capacitor Cs, the drive current doesnot change during the hold mode.

In the configuration shown in FIG. 3A, the current of the pixel 31depends on the absolute resistance of resistor 32, which is notdesirable due to possible inherent inaccuracy and poor thermal stabilityof integrated resistors. FIG. 4 presents an architecture, according toanother embodiment of the invention that addresses this by implementinga reference resistor 42 and an external data current source 41. Thereference resistor 42 is made with the same material as integratedresistors and formed on the display substrate. This enhances thetemperature stability of the circuit. The programmed level of the drivecurrent for this circuit is:

$\begin{matrix}{{I_{OLED} = {{Iinp}\;\frac{Rr}{Rf}}},} & (2)\end{matrix}$where Rr is the resistance of the reference resistor 42, and Rf is theresistance of the feedback resistor 32. The above equation indicates aconsiderable improvement in the accuracy of the programming currentbecause of insensitivity of the resistance ratio to the temperaturevariations.

A current pixel drive circuit according to another embodiment of theinvention and a section of the column driver circuitry are shown in FIG.5. The circuit is similar to the circuit of FIG. 3A however, in thecircuit of FIG. 5, the cathode of OLED 51 is common and is connected toa negative power supply potential Vss. As a result, the cathode of theOLEDs is not patterned.

The anode of OLED 51 is connected to the source terminal of transistor56. The feedback resistor 32 is connected between the drain terminal oftransistor 56 and ground node. The voltage level of the select lineduring the writing mode should be high enough to guarantee thattransistor 54 is in “on” state for the entire output current range. Thefeedback line in this configuration is connected to the non-invertinginput of the difference amplifier 33 to provide a negative feedback.

FIGS. 6A, 7 and 8 illustrate pixel drive circuit, according to otheraspects of the invention wherein p-channel MOS transistors are used.

FIG. 6A shows a pixel circuit, according to another embodiment of theinvention. The feedback switch use transistor 68 is p-channeltransistor. The circuit is similar to the circuit of FIG. 5, however theimplementation of the PMOS transistor requires an additional selectline. FIG. 6B shows corresponding waveforms for select line A and selectline B. The advantage of this circuit over the circuit of FIG. 5 is thelower voltage swing for the select lines that is required.

FIG. 7 shows a pixel circuit according to another embodiment of theinvention. The transistors 76 and 74 are p-channel transistors and thetransistor 78 is an n-channel transistor. As an embodiment of FIG. 7this circuit also has two select lines marked as A and B having reducedvoltage swing.

In the pixel circuit shown in FIG. 8, all of the transistors arep-channel transistors. Here the anode of the OLED 51 is connected to thedrain terminal of the transistor 76 and the common cathode electrode ofthe OLED 51 is connected to the negative power supply potential Vss.

FIGS. 9 and 10 show configurations of the pixel circuits according toalternative embodiments of the invention. In these pixel circuits, thecurrent driving circuitry is based on a current mirror architecture,i.e. transistors 96 and 97 and 108 and 110. The current level of thesignal current and the current level of the drive current areproportional. In the circuit of FIG. 9, all transistors are n-channeltransistors and in the circuit of FIG. 10 all transistors are p-channeltransistors.

In FIG. 10, the feedback resistor 32 is connected between the drainterminal of transistor 106 and common ground. The gate terminals of thetransistors 106 and 110 are connected. In the circuit of FIG. 9, thecathode electrode of OLED 31 is connected to the drain terminal oftransistor 97, the anode is common and the transistor 97 is the drivingtransistor and is connected to OLED 31. In the circuit of FIG. 10, thecathode of the OLED 51 is common and the anode is connected to the drainterminal of the transistor 110.

During writing mode, the transistors 104 and 108 are in an “on” state,thus the transistor 106 along with feedback resistor 32 and externalcontrol unit (the difference amplifier 33) form a feedback loop. Thetransistor 110 does not directly take part in the feedback loop, butsince the transistors 110 and 106 have same gate-source voltage, thecurrent of the transistor 110 is proportional to the current of thetransistor 106. The ratio of current through transistors 110 to 106 isdetermined by the aspect ratios of these transistors. In these circuits,the feedback resistor 32 and the OLED 31 of FIGS. 9 and 51 of FIG. 10,are not in the same current path thus a higher lifetime is expected.

Several methods have been used to reduce the charge injection and clockfeed-through effects in integrated circuits. As the simplest approach, adummy transistor that is driven by the inverse signal of the select lineconnected to the gate of driving transistor can reduce both chargeinjection and clock feed-through errors caused by the driving switch.The drain and source terminals of the dummy transistor are connected tothe gate of the driving transistor. FIG. 11 shows an example of suchmodification for the embodiment of FIG. 3. The width of dummy transistor118 is half of the width of driving transistor 116. It will be apparentto one skilled in the art that the width of the dummy transistor 118 canbe any appropriate size.

FIG. 12 is a schematic circuit diagram of another embodiment of a pixelcircuit, column control unit and a reference cell according to thepresent invention. Here, the implemented luminance feedback improves thelinearity of the video signal—light output characteristics, and alsoprovides a compensation for brightness instability caused by instabilityof the organic material, ageing, temperature changes, or otherenvironment stresses. The compensating circuit with luminance feedbackincludes a resistor R1, a difference amplifier 121, and aNMOS-transistor 122, which are parts of the control unit, and theelements of the reference cell 123 including an OLED 124, and photodiode125. The photodiode 125 is optically coupled to the reference OLED 124to form a feedback current signal in response to emitted light. Thecircuit is balanced when the input current passing through the resistorR1 is equal to a feedback current generated by the photodiode 125. Thecurrent flowing through OLED 124 via transistor 122 and resistor 42 isan input signal for next stage of the device, which is the same as theembodiment of FIG. 4.

FIG. 13 is a schematic diagram of an alternative embodiment of theembodiment of FIG. 4. In this embodiment, diodes D1 and D2 are used inplace of feedback resistor RI and reference resistor R2 of FIG. 3,respectively. As circuit functionality with reasonably low error in theprogrammed current level requires a good match between the referencediode and pixel diodes, the fabrication technology must be efficient forfabrication of the diode array with reproducible forward current-voltagecharacteristics.

A schematic diagram of a circuit according to another embodiment of theinvention is shown in FIG. 14. This circuit implements a common cathodeOLED array configurations. In the writing mode, the input current froman external current data source 41 creates a voltage drop across areference OLED 141. A difference amplifier 33 in negative feedbackconnection is designed to hold the same voltage level on a pixel OLED142. During the hold mode, the current with a programmed current levelflows through both the transistor 146 and the OLED 142 due to thevoltage stored across the capacitor Cs.

Although the exemplary embodiments of the present invention aredescribed in conjunctions with OLEDs, it is also contemplated othersimilar display elements such as a light emitting diode (LED) could beused in other embodiments.

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A pixel circuit for use in a display, the displaycomprising a plurality of pixels with each pixel having a select line, asignal line, and a feedback line, each of the pixels comprising: a lightemitting element; a drive part for providing a drive current to thelight emitting element, the drive part having a storage capacitor, aswitch use transistor, a drive use transistor, and a feedback diode; theswitch use transistor having a gate terminal connected to the selectline, a first terminal connected to the signal line, and a secondterminal connected to the storage capacitor, the drive use transistorhaving a gate terminal connected to the second terminal of the switchuse transistor, a first terminal connected to the light emittingelement, and a second terminal connected to the feedback diode, and thefeedback switch having a gate connected to the select line, a firstterminal connected to the drive use transistor, and a second terminalconnected to the feedback line.
 2. The pixel circuit according to claim1, wherein the storage capacitor is connected to ground or to the firstterminal of the feedback switch.
 3. The pixel circuit according to claim1, wherein the light emitting element is connected between the secondterminal of the drive use transistor and a reference potential.
 4. Thepixel circuit according to claim 1, wherein the diode is connectedbetween the second terminal of the drive use transistor and apredetermined potential.
 5. The pixel circuit according to claim 1,wherein said light emitting element is an organic light emitting diode.6. The pixel circuits according to claim 1, wherein the feedback switchis a transistor, and wherein the transistors are insulating gate typefield effect transistors that comprise n-channel and p-channel typetransistors.
 7. The pixel circuit according to claim 1, furthercomprising: a difference amplifier having an output connected to thesignal line, a non-inverting input connected to the feedback line, andan inverting input connected to a reference diode having a matchedforward current-voltage characteristic with that of the feedback diode.8. The pixel circuit according to claim 7, wherein the differenceamplifier is a high-gain, low offset difference amplifier.
 9. The pixelcircuit according to claim 7, further comprising a current sourcesupplying an input current to the non-inverting input of the differenceamplifier.
 10. The pixel circuit according to claim 7, wherein the samecorresponding terminal of the reference diode and of the feedback diodeis connected to a negative power supply potential.
 11. The pixel circuitaccording to claim 10, wherein the storage capacitor is connected to thenegative power supply potential.
 12. The pixel circuit according toclaim 1, further comprising: a difference amplifier having an outputconnected to the signal line, a non-inverting input connected to thefeedback line, and an inverting input connected to a reference diode,wherein the reference diode and the feedback diode are matched.
 13. Apixel circuit for use in a display, the display comprising a pluralityof pixels each pixel connected to a select line, a signal line, and afeedback line, each of the pixels comprising: a light emitting element;a drive part for providing a drive current to the light emittingelement, the drive part including: a storage capacitor; a switch usetransistor; a drive use transistor connected to the light emittingelement; a feedback transistor having a gate connected to the selectline, a first terminal connected to the drive use transistor and to thestorage capacitor, and a second terminal connected to the feedback line;the switch use transistor having a gate terminal connected to the selectline, a first terminal connected to the signal line, and a secondterminal connected to the storage capacitor, the drive use transistorhaving a gate terminal connected to the second terminal of the switchuse transistor, a first terminal connected to the light emittingelement, and a second terminal connected to a reference potential. 14.The pixel circuit according to claim 13, wherein the light emittingelement is an organic light emitting device (OLED), the pixel circuitfurther comprising: a difference amplifier having an output connected tothe signal line, a non-inverting input connected to the feedback line,and an inverting input connected to a reference OLED such that the OLEDand the light emitting element form a common cathode OLED arrayconfiguration.
 15. The pixel circuit according to claim 14, wherein thedifference amplifier is a high-gain, low offset difference amplifier.16. The pixel circuit according to claim 14, further comprising acurrent source supplying an input current to the non-inverting input ofthe difference amplifier.
 17. The pixel circuit according to claim 14,wherein the same corresponding terminal of the reference OLED and of thelight emitting element is connected to a negative power supplypotential.
 18. A method of driving a plurality of light emittingelements arranged in a column, comprising: selecting one pixel of aplurality of pixels in the column via a select line, each of the pixelsincluding a light emitting element and a current driving circuit havinga storage capacitor and a switch use transistor; establishing, in acontrol unit external to the selected pixel, the desired brightness of areference diode of the control unit according to an external referencecurrent supplied to a current source of the control unit; converting inthe control unit the reference current flowing through the referencediode into a corresponding voltage level; transmitting, by a differenceamplifier in the control unit, the voltage level from the control unitto the selected pixel; converting in the selected pixel the voltagelevel into a drive current and generating a feedback voltage signalrepresenting a drive current level to form a negative feedback loopbetween the selected pixel and the difference amplifier; the differenceamplifier adjusting the voltage level in response to the feedbackvoltage signal from the selected pixel to establish a drive currentsubstantially equal to the reference current; storing the adjustedvoltage level in the selected pixel; and driving the light emittingelement of the selected pixel with the drive current in accordance withthe adjusted voltage level, wherein the reference diode in the controlunit and the light emitting element in the selected pixel have matchedfabrication characteristics.
 19. The method according to claim 18,wherein the reference diode is an organic light emitting diode.